This is the eighth article in our series about Power Design Pro™ software. In this issue, you’ll learn about load shedding features and how these assist engineers with “what-if” scenarios for generator sizing and design.
Power Design Pro™ has a unique feature called load shedding that’s identifiable by multiple check boxes on the load summary page. There is one check box for each load sequence, located within the given sequence’s subtotal information. When this box is checked, the program automatically zeros out the subtotaled values for that particular sequence. Extremely useful for various “what-if” analyses, this feature allows end-users to easily remove and/or re-apply various loads that are grouped together without having to delete and re-enter them.
Generating point for many customers is a desire to back up all the loads at a given site. As the loads are analyzed, the resulting generator solution may be larger than the planned budget. The load shed feature allows the user to easily see the effects of removing – or adding back – large discrete loads. A few simple interlocking control contacts could easily prevent some large motors from running while on generator power. Many times simply evaluating the removal of a few large loads and properly sequencing them can allow the generator to pick-up the entire building while still meeting budget constraints.
Analyzing Fire Pump Sizing
On the surface, fire pumps don’t seem like they would fit into a load shed decision. However, fire pumps are often integrated into applications with generators that also carry optional standby circuits. These two load circuits should be viewed as mutually exclusive. If the fire pump controller switches to generator power, the site’s optional standby circuits, fed from the generator, would typically be shunt tripped off. From a fire fighter’s perspective, optional standby loads should not be energized while extinguishing a fire.
In the below example, carrying both optional loads in Group 1 and the fire pump in Group 2 would require a 350kW generator (figure 2). But by using the load shed feature, the user can evaluate the generator size needed to run each load grouping separately. Of the two, the larger would determine the minimum size for the generator. In this example, the optional standby loads in Group 1 require a 200kW generator with no upsize on the alternator. The fire pump in Group 2 requires a 175kW generator with an upsized 275 alternator for voltage stiffness (figure 3). Combining this information together, the application could be served by a 200kW generator with a 275 alternator. This would typically represent $52,000 in fully installed savings from the initial 350 kW solution.
Evaluating N+1 Redundancy
When designing redundant power solutions, load shedding schemes are often implemented to maintain system integrity. Power Design Pro provides the ability to shed loads that are entered into the program and allow the user to evaluate the effects of running those loads versus any user selected generator configuration. The user can manually change the generator sizing from 3x500 kW to 2x500 kW (figure 4) then use the load shedding feature to shed off targeted load circuits. The remaining circuits can now be evaluated for performance with just two generators running versus the initially scoped three generators. The generators’ performance can be evaluated based on load level, voltage and frequency transient capability, and calculated harmonic distortion levels.
The Power Design Pro load shed feature makes performing “what-if” analyses extremely easy. Simply checking a box removes the targeted loads from the sizing solution, providing the ability to evaluate impact. This functionality is very helpful when trimming loads from the scope of supply, sizing applications with fire pumps and optional standby circuits, or evaluating the effects of generator capacity in N+1 mission critical applications.
If you would like more information about Power Design Pro™ and its many capabilities, please visit the Power Design Pro™ page.